Single-fireball tangentially-firing boiler for the burning of anthracite

ABSTRACT

A tangentially-fired furnace for the burning of anthracite is disclosed. The furnace may contain a boiler with a chamber having four corners. Four burner groups located at the four corners may be configured to eject pulverized coal flow into the chamber for combustion, in order to form a single fireball substantially at the center of the chamber during combustion. Each burner group may contain a first burner which includes primary-air/rich-portion nozzles for ejecting rich-portion coal flow into a lower section of the chamber, and a second burner which includes primary-air/thin-portion nozzles for ejecting thin-portion coal flow into a higher section of the chamber.

CROSS-REFERENCE

This application is a continuation application of an International Application No. PCT/CN2012/073968, filed on Apr. 13, 2012, which claims priority to a Chinese Patent Application No. 201110358272.5, filed on Nov. 14, 2011, both of which are hereby incorporated by reference in their entireties, including any appendices or attachments thereof, for all purposes.

TECHNICAL FIELD

The present disclosure is related to a pulverized coal combustion system. Specifically, the present disclosure is related to a single-fireball tangentially-fired furnace for the burning of anthracite.

BACKGROUND

Currently, the ascertained coal reservation in China is about 640 billion tons, among which, the low-volatile anthracite accounts for about 14.6% of the total amount. About 3% of the power plants in China use anthracite to generate electricity, and this figure may increase in the future. Anthracite has the benefits of low volatile, low hydrogen content, high ignition temperature, and slow flame propagation speed. However, if not burnt correctly, the anthracite may cause unstable combustion for a boiler that is operating under a low load. Also, low-grade anthracite may cause low burning efficiency or flameout for a boiler that is operating under a heavy load.

The commonly used boilers in China include the W-flame boiler, the conventional tangentially-fired boiler, and the opposite-wall cyclone-fired boiler. The maximum power-generating capacity for the W-flame boiler is about 600 MWs (mega-watts). The maximum power-generating capacity for the conventional tangentially-fired boiler or the opposite-wall cyclone-fired boiler is about 300 MWs. None of the conventional tangentially-fired boiler or the cyclone-fired boiler can achieve a capacity that exceeds 600 MWs.

SUMMARY

Accordingly, various aspects of present disclosure may provide a tangentially-fired furnace for the burning of anthracite. The furnace may include a boiler with a chamber having four corners. Four burner groups located at the four corners may be configured to eject pulverized coal flow into the chamber during combustion, in order to form a single fireball substantially at the center of the chamber. Each burner group may contain a first burner which includes primary-air/rich-portion nozzles for ejecting rich-portion coal flow into a lower section of the chamber, and a second burner which includes primary-air/thin-portion nozzles for ejecting thin-portion coal flow into a higher section of the chamber.

BRIEF DESCRIPTION OF THE DRAWINGS

To clarify the present disclosure, the accompanying drawings for the various embodiments are briefly described below.

FIG. 1 illustrates a horizontal cross-sectional view of an exemplary embodiment of an ultra-supercritical boiler;

FIG. 2 illustrates a vertical cross-sectional view of an exemplary embodiment of an ultra-supercritical boiler;

FIG. 3 illustrates an embodiment of a single-fireball tangentially-firing boiler system having an anthracite boiler and a corresponding pulverizer;

FIG. 4 illustrates a horizontal cross-sectional view of a boiler capable of distributing rich-portion and thin-portion pulverized coal flows; and

FIG. 5 illustrates a vertical cross-sectional view of a boiler capable of distributing rich-portion and thin-portion pulverized coal flows, all according to various embodiments of the present disclosure.

DETAILED DESCRIPTION

To clarify the present disclosure, the following description and the accompanying drawings illustrate the embodiments of a single-fireball tangentially-fired boiler for the burning of anthracite. The boiler may be configured to change the distributing of the rich-portion and thin-portion pulverized coal in its burning chamber, in order to reach and maintain the thermal power output requirement of the boiler. Specifically, the boiler may adjust the concentration of the pulverized coal in the coal flow, thereby allowing a lower section of the burning chamber to achieve a higher q_(Hr) heat load. Such an approach may ensure that the lower section of the burning chamber can not only reach the combustion temperature required for the burning of the anthracite, but also guarantee the timely and stable combustion of the anthracite coal flow when the boiler is operating under a low load.

The manufacturers of power plant boilers started their design and manufacturing of anthracite boilers since the 1970s. With the capacities of the boilers starting to approach a range between 600 MWs and 1300 MWs, the thermal parameters of these boilers may be totally different compared to the 125 MW-grade or 300 MW-grade boilers. For these high-capacity boilers, the furnace “volume heat load” parameter q_(v) may be lower, and the pulverized coal flow may spend longer time in the boiler chamber. These characteristics of the high-capacity boilers may allow the anthracite to burn properly. However, the “boiler sidewall surface heat load” parameter q_(Hr) (a measurement calculated by dividing the heat load of the boiler with the boiler internal sidewalls' surface areas) may also be lower. Even though the “intersectional heat load” parameter q_(f) (to measure the average amount of heat load in the boiler chamber, calculated based on the intersectional area of a specific region in the boiler chamber) may remain high, the total heat absorption rate for the water-cooled walls of these high-capacity boilers may also be high. This may cause the lowering of the combustion temperature in the boiler chamber, and affect the timely and stable combustion of the pulverized anthracite.

In order to increase a boiler's power-output capacity to a range between 800 MWs to 1300 MWs, the number of the pulverized coal nozzles that are connected to a single coal pulverizer may need to be increased for about 50% to 100%, since the thermal power that can be generated by a single pulverized coal nozzle is somewhat limited. Compared to a 300 MW-600 MW range boiler, which may have four nozzles that are connected with a single pulverizer, the number of nozzles that are connected with a single pulverizer in a high-capacity boiler may be increased to 6 or 8.

For a super-critical boiler having a power-output capacity of 1000 MWs and equipped with 4 to 6 medium-grinding-speed or dual-inlet-dual-outlet pulverizers, a single pulverizer may be associated with 8 nozzles. Therefore, the total number of nozzles in a super-critical boiler may reach 48. Under a direct-flow tangentially-firing burner arrangement, each corner of the boiler chamber may be equipped with 12 of these 48 nozzles. When these 12 nozzles are vertically distributed to 2 to 3 burners, then a distance from the top nozzle of the top burner to the bottom nozzle of the bottom burner may be very far. This may result in the lowering of the boiler sidewall surface heat load q_(Hr) and the lowering of the combustion temperature in the boiler chamber, and may greatly affect the timely and stable combustion of anthracite.

FIG. 1 illustrates a horizontal cross-sectional view of an exemplary embodiment of an ultra-supercritical boiler, which may have a power-output capacity of about 1000 MWs. In FIG. 1, the boiler may have a boiler body 1′, and may be equipped with six pulverizers 3′ (denoted A, B, C, D, E, and F in FIG. 1). The boiler may have a chamber 2′, which is formed by four water-cooled walls 9′. A corresponding burner group may be installed in each corner of the chamber 2′. During operation, the pulverized-coal airflows ejected from the nozzles of the four burner groups in these four corners may form a fire ball 11′ (that is substantially circular in a sectional view) around approximately the center of the chamber 2′.

FIG. 2 illustrates a vertical cross-sectional view of an exemplary embodiment of an ultra-supercritical boiler. The vertical cross-section may be formed by cutting vertically across a line through the boiler, as illustrated by e.g., II-II line in FIG. 1 (the resulting cross-section may be viewed in a direction indicated by arrows 101 and 102 of FIG. 1). Likewise, the cross-sectional view in FIG. 1 may be formed by cutting horizontally across a line through the boiler, as illustrated by e.g., I-I line in FIG. 2 (the resulting cross-section may be viewed in a direction indicated by arrows 201 and 202 of FIG. 2).

In FIG. 2, each burner group 210 may have multiple (e.g., three) burners 10′ vertically installed in one corner of the chamber 2′. The burners 10′ may be arranged from top to down following a fixed interval (that is, any two adjacent burners 10′ are separated by a fixed distance). Each burner 10′ may have multiple (e.g., four) primary-air pulverized-coal nozzles 6′ and multiple (e.g., six) secondary-air pulverized-coal nozzles 8′. The primary-air nozzles 6′ and the secondary-air nozzles 8′ may be arranged with one secondary-air nozzle 8′ placed next to one primary-air nozzle 6′. For example, three secondary-air nozzles 8′ and two primary-air nozzles 6′ may form a first burner unit, and the next three secondary-air nozzles 8′ and the next two primary-air nozzles 6′ may form a second burner unit. Thus, the primary nozzles 6′ in the specific burner group (denoted burner group 1) that is installed in the first corner may be marked, from top to down, using the following markings: A1-1, A1-2, B1-1, B1-2, C1-1, C1-2, D1-1, D1-2, E1-1, E1-2, F1-1, and F1-2. Similarly, the primary-air nozzles 6′ in the specific burner group (denoted burner group 4) installed in the fourth corner may be marked as A4-1, A4-2, all the way to F4-1 and F4-2, respectively. The above marking scheme may be applied to the burner groups and the nozzles in the rest of corners.

In addition, each pulverizer 3′ may have multiple (e.g., four) pulverized coal pipes 5′ connected to its outlet. Each of the pipes 5′ may pass through one of the distributors 4′ configured to separate and distribute the pulverized coal to two primary-air nozzles 6′ located in one corresponding burner group in one corner of the chamber 2′. For example, the pulverizer A may be connected with eight primary-air nozzles 6′ marked A1-1, A1-2, A2-1, A2-2, A3-1, A3-2, A4-1, and A4-2 (A2-1, A2-2, A4-1 and A4-2 are not shown in FIG. 2). In other words, each of the pulverizers 3′ may have four pipes 5′ connected with four corresponding distributors 4′ located in the four corners of the chamber 2′. Each distributor 4′ may further have two pipes connecting the specific distributors 4′ with two of the twelve primary-air nozzles 6′ in one of the four corners.

The nozzles in a specific corner may be connected with one of the pulverizers. For example, at corner 1, the primary-air nozzles A1-1 and A1-2 may be connected to the pipes and the distributor that are connected with the pulverizer A. The primary-air nozzles B1-1 and B1-2 are connected to the pipes and the distributor that are connected with the pulverizer B. And the rest of the primary-air nozzles in corner 1 may be connected in similar fashions. Still, in this arrangement, the distances among the primary nozzles may be large, which in turn may lead to a boiler sidewall surface heat load q_(Hr) that is too low for the efficient burning of anthracite.

For the anthracite boilers with 50 MWs, 125 MWs, or 300 MWs capacities, most of them may use the above system to deliver the hot air and pulverized-coal flow to the mid-section of the boiler. Specifically, when mixing the hot air with the coal flow, the above system may achieve a temperature between 200-250° C., and a primary-air delivery ratio of 14-15%. It can also remove the exhaust gas (which may contain water) from the primary air and send the exhaust gas back to the upper chamber of the boiler, in order to reduce the pulverized coal combustion temperature. Thus, a mid-sectional hot-air and coal-dust delivery system as described above may be the key in efficient burning of the anthracite for the 50-300 MW boiler.

To use a mid-section hot-air and coal-flow delivery system, since many pulverizers can generate up to 50 ton/hour of pulverized coal, a typical 600 MWs or above boiler may require 6 to 8 of such pulverizers. Thus, such a delivery configuration may be complicated in design, and require large space for installation. Therefore, this delivery configuration may not be the best choice for a boiler that has more than 600 MWs capacity.

FIG. 3 illustrates an embodiment of a single-fireball tangentially-firing boiler system having an anthracite boiler and a corresponding pulverizer. In FIG. 3, a boiler 10, which may be a single-fireball tangentially-firing boiler, may be connected with one or more pulverizers 30. The pulverizer 30 may be a mid-speed dual-inlet-dual-outlet direct flow pulverizer which can be installed with a distributor 40 that can separate the pulverized coal dust into a rich-portion and a thin-portion coal flow. One end of a pipe 50 may be connected with an outlet of the pulverizer 30. The other end of the pipe 50 may be connected with a specific distributor 40 that can separate and delivery the pulverized coal into a rich-portion coal flow and a thin-portion coal flow. The rich-portion coal flow (which may contain e.g., about 80% of the pulverized coal coming from the pulverizer 30 and e.g., about 50% of the primary air) may contain a higher concentration of pulverized coal than the thin-portion coal flow (which may contain e.g., about 20% of the remaining pulverized coal coming from the pulverizer 30 and e.g., about 50% of the primary air).

The rich-portion coal flow and the thin-portion coal flow may be delivered to one or more “primary-air/rich-portion” nozzles 60 and one or more “primary-air/thin-portion” nozzles 70 via the pipe 42 and pipe 41, respectively. Once ejected into the chamber 20 of the boiler 10 via the one or more primary-air/rich-portion nozzles 60, the rich-portion coal flow may be burnt in a rich combustion area 21. Similarly, the thin-portion coal flow may be ejected into the thin combustion area 20 by the one or more primary-air/thin-portion nozzles 70 for combustion. Thus, during operation, boiler 10 may have a rich combustion area 21 forming in its lower part of the chamber 20, and a thin combustion area 20 forming above the rich combustion area 21.

FIG. 4 and FIG. 5 may be used to illustrate an exemplary embodiment of a single-fireball tangentially-firing boiler capable of distributing rich-portion and thin-portion pulverized coal flows, as shown above in FIG. 3. Specifically, FIG. 4 may illustrate a horizontal cross-sectional view of the boiler formed by cutting across a line through the boiler, as illustrated by e.g., I-I line in FIG. 5 (the resulting cross-section may be viewed in a direction indicated by arrows 501 and 502). Likewise, the cross-sectional view in FIG. 5 may be formed by cutting vertically across a line through the boiler, as illustrated by e.g., II-II line in FIG. 4 (the resulting cross-section may be viewed in a direction indicated by arrows 401 and 402).

In FIG. 4, the tangentially-firing boiler may have a boiler body 1, and may be equipped with multiple (e.g., six) pulverizers 3 (denoted A, B, C, D, E, and F in FIG. 4). The boiler may have a chamber 2, which is formed by four water-cooled walls 9. One corresponding burner group may be installed in each of the four corners in the chamber 2. The pulverized-coal dusts ejected from the nozzles of the four burner groups and into the chamber may form a fire ball 11, when combusted, around approximately the center of the chamber 2. For better illustrational purposes, the lower left corner in FIG. 4 may be marked as corner 1, and the rest of the corners may be marked, clockwise, as corner 2, corner 3, and corner 4.

In FIG. 5, for each burner group 510, multiple (e.g., two) burners 10 may be vertically installed in a specific corner of the chamber 2. The multiple burners 10 may be arranged top-down and spaced-out (e.g., with a fixed distance between the multiple burners 10). A first burner 10, which may be referred to as “primary-air/rich-portion” burner, may be positioned at the lower part of the chamber 2. The first burner 10 may further contain multiple (e.g., seven) secondary-air nozzles 8 and multiple (e.g., six) primary-air/rich-portion nozzles 6. The primary-air/rich-portion nozzles 6 and the secondary-air nozzles 8 may be intertwine-distributed, with one secondary-air nozzle 8 next to at least one primary-air/rich-portion nozzle 6.

A second burner 520, which may be referred to as “primary-air/thin-portion” burner, may be positioned above the first burner 10 and at the upper part of the specific corner in the chamber 2. The second burner 520 may further contain multiple (e.g., seven) secondary-air nozzles 8 and multiple (e.g., six) primary-air/thin-portion nozzles 7. The primary-air/thin-portion nozzles 7 and the secondary-air nozzles 8 may be intertwine-distributed, with one secondary-air nozzle 8 next to at least one primary-air/thin-portion nozzle 7.

For better illustrational purposes, the first burner 10, which is installed at the lower part of corner 1, may have six primary-air/rich-portion nozzles 6 marked as A1-1, B1-1, C1-1, D1-1, E1-1 and F1-1, respectively. The second burner 520, which is located at the upper part of corner 1, may have seven primary-air/thin-portion nozzles 7 marked as A1-2, B1-2, C1-2, D1-2, E1-2 and F1-2, respectively. Likewise, the six primary-air/rich-portion nozzles 6 of the first burner group installed on the lower part of the fourth corner may be marked as A4-1, B4-1, C4-1, D4-1, E4-1, and F4-1, respectively; and the six primary-air/thin-portion nozzles 7 of the second burner group installed on the upper part of the fourth corner may be marked as A4-2, B4-2, C4-2, D4-2, E4-2, and F4-2, respectively. For the rest of corners in the chamber 2, the primary-air/rich-portion nozzles 6 and the primary-air/thin-portion nozzles 7 may be similarly marked.

In some embodiments, each pulverizer 3 may have four coal dust pipes 5 connected to its outlets. Each of the pipes 5 may pass through one of the distributors 4 configured to separate and distribute the rich-portion and the thin portion coal flows to one specific primary-air/rich portion nozzle 6 and one specific primary-air/thin-portion nozzle 7 that are located in the same corner of the chamber 2.

Taking the example of a pulverizer 3 which is marked with “A” in FIG. 5, the pulverized coal generated by the pulverizer “A” may first be delivered to the four distributors 4. Each of the four distributors 4 may then separate the pulverized coal it received into a rich portion coal flow and a thin portion coal flow. Thus, the rich-portion coal flows from the four distributors 4 may be transported via pipes to the four first burners 10 installed in the lower part of the four corners of the chamber 2, and may be ejected out of the four primary-air/rich-portion nozzles 6 (e.g., A1-1, A2-1, A3-1, and A4-1), respectively. In other words, the four primary-air/rich-portion nozzles 6, which are used for ejecting rich-portion coal flows originated from the pulverizer “A”, are selected from all the primary-air/rich-portion nozzles 6 in the four first burners 10 for being located on a same horizontal flat plane.

Likewise, the thin-portion coal flows from the four distributors 4 may be transported via pipes to the four second burners 510 installed in the upper part of the four corners of the chamber 2, and may be ejected out of the four primary-air/thin-portion nozzles 7 (e.g., A1-2, A2-2, A3-2 and A4-2), respectively. In other words, the four primary-air/thin-port nozzles 7, which are used for ejecting thin-portion coal flows originated from the pulverizer “A”, are selected from all the primary-air/thin-portion nozzles 7 in the four second burners 520 for being located on a same horizontal flat plane. The other pulverizers 3, including pulverizer “B”, “C”, “D”, “E”, and “F”, may be similarly connected with the corresponding pipes 5, distributors 4, first burners 10, second burners 520, primary-air/rich-portion nozzles 6, and primary-air/thin-portion nozzles 7, as described above. For example, the pulverized coal generated by the pulverizer “B” may ultimately be ejected from the nozzles that have markings that start with “B.” Please note that for clarification purposes, some of the pipes, distributors, and connections that are necessary for connecting the various components are omitted in FIG. 5.

Based on the above distribution setup, the rich-portion coal flows generated by the pulverizers 3 and separated by the distributors 4 may be ejected via the various primary-air/rich-portion nozzles 6 to the lower section of the chamber 2, and forming a rich combustion area in the lower section of the chamber 2. Likewise, the thin-portion coal flows generated by the pulverizers 3 and separated by the distributors 4 may be ejected via the various primary-air/thin-portion nozzles 7 to the upper section of the chamber 2, and forming a thin combustion area in the upper section of the chamber 2. Thus, when the overall heat load of the boiler can be satisfied, increasing the concentration of the pulverized coal in the rich combustion area of the chamber may increase the boiler sidewall surface heat load q_(Hr). Such an approach may ensure that the boiler can reach the temperature required for stable combustion of anthracite. It can also ensure the stabilization and the timely ignition of the coal flow in the chamber, and the stable combustion for the boiler when operating under low work load or when no ignition fuel is used.

Thus, the disclosed embodiments of a single-fireball tangentially-fired burner for the burning of anthracite may have the following advantages. First, by adopting a medium-grinding-speed or dual-inlet-dual-outlet pulverizer with a distributor that can separate rich portion and thin portion coal flows from the pulverized coal, the tangentially-fired burner may achieve a rich combustion area and a thin combustion area in its chamber. Further, the primary-air/rich-portion coal flow may have a better coal-to-air ratio and a better primary air ratio than the coal flow formed in burners that use a mid-section hot-air and coal-flow delivery system.

In some embodiments, the present disclosure provides a mechanism that sends about 50% of the primary air (as much as half of which may be water) into the chamber via the primary-air/thin-portion nozzles that are located at the top of the chamber. Even though the mixing temperature of this primary-air/thin-portion coal flow may be lower than the temperature in a chamber that uses a mid-section hot-air and coal-flow delivery system, based on calculation, the ignition temperature remains the same in both approaches. Thus, such an approach may ensure the stable ignition of rich portion coal flow.

In some embodiments, when the efficient burning of coal is ascertained, and when a required distance from a primary-air nozzle located on the top of one corner to the bottom of a chamber exit is satisfied, the distance between the first burner and the second burner may be adjusted, in order to adjust a distance from a specific primary-air/thin-portion nozzle (located at the top of the one of the corners) to a primary-air/rich-portion nozzle (located at the bottom of the same corner) to an optimal range. Such an adjustment may help reducing the emission of nitrogen oxide by the boiler.

In some embodiments, the primary-air/rich-portion nozzles and the primary-air/thin-portion nozzles may be configured with perimeter airflow (airflow generated by secondary-air nozzles and surrounding the primary-air flows), and the nozzles that eject secondary air may be adjusted to provide an offset airflow (airflow generated by secondary-air nozzles and in-between the flames and the heat absorption surfaces), in order to achieve an ideal combustion which may be referred to as “airflow-surrounding-pulverized coal” combustion. Under this ideal combustion, the phenomenon of pulverized coal sticking to the sidewalls, slag buildup in the chamber, and corrosions caused by high temperatures may be prevented or reduced. The ideal combustion may also allow the boiler to easily switch to a different type of coal.

Thus, methods and systems for a tangentially-fired furnace have been described. Although the present disclosure has been described with reference to specific exemplary embodiments, it will be recognized that the disclosure is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. 

We claim:
 1. A tangentially-fired furnace for the burning of anthracite, comprising: a boiler (1) having a chamber (2) surrounded by four water-cooled walls (9), wherein the four water-cooled walls intersect one-another to form four corners in the chamber (2); and four burner groups located at the four corners of the chamber (2) respectively, for ejecting pulverized-coal fuel to be ejected from the burner groups to combust and form a single fireball substantially at the center of the chamber (2), wherein each of the four burner groups contains a first burner and a second burner vertically located in a corresponding corner of the four corners, the first burner is a primary-air/rich-portion burner located in a lower part of the chamber (2), ejecting a first concentration of pulverized-coal fuel to form a rich combustion area in the chamber (2), the second burner is a primary-air/thin-portion burner located in an higher part of the chamber (2), ejecting a second concentration of pulverized-coal fuel to form a thin combustion area in the chamber (2), and the first concentration is higher than the second concentration.
 2. The tangentially-fired furnace as recited in claim 1, wherein the first burner includes, in a vertical direction, one or more primary-air/rich-portion nozzles (6) and a first set of secondary-air nozzles (8), and the primary-air/rich-portion nozzles (6) are intertwined with the first set of secondary-air nozzles (8).
 3. The tangentially-fired furnace as recited in claim 2, wherein the second burner includes, in the vertical direction, one or more primary-air/thin-portion nozzles (7) and a second set of secondary-air nozzles (8), and the primary-air/thin-portion nozzles (7) are intertwined with the second set of secondary-air nozzles (8).
 4. The tangentially-fired furnace as recited in claim 1, wherein the first burner contains seven secondary-air nozzles (8) and six primary-air/rich-portion nozzles (6), and the second burner contains additional seven secondary-air nozzles (8) and six primary-air/thin-portion nozzles (7).
 5. The tangentially-fired furnace as recited in claim 1, further comprising: a pulverizer (3) coupled with one of the four burner groups, wherein the pulverizer (3) is configured to generate pulverized coal from anthracite, and contains one or more outlets for connecting with one or more distributors (4) via one or more pipes (5), each of the distributors (4) is configured to separate the pulverized coal fuel into a rich-portion and a thin-portion, the rich-portion is transported to the first burner of the one of the four burner groups, the thin-portion is transported to the second burner of the one of the four burner groups, the rich-portion is ejected via a primary-air/rich-portion nozzle (6) located in the first burner, and the thin-portion is ejected via a primary-air/thin-portion nozzle (7) located in the second burner.
 6. The tangentially-fired furnace as recited in claim 5, wherein the pulverizer (3) is connected with four of the distributors (4) generating four rich-portion coal flows and four thin-portion coal flows, the four rich-portion coal flows are transported to the corresponding four primary-air/rich-portion nozzles (6) in the four first burners of the four burner groups, the four thin-portion coal flows are transported to the corresponding four primary-air/thin-portion nozzles (7) in the four second burners of the four burner groups, the four primary-air/rich-portion nozzles (6) are located on a first horizontal flat plane, and the four primary-air/thin-portion nozzles (7) are located on a second horizontal flat plane.
 7. The tangentially-fired furnace as recited in claim 6, wherein the rich-portion coal flow contains about 80% of the pulverized coal generated by the pulverizer (3) and about 50% of primary air, the thin-portion coal flow contains about 20% of the pulverized coal generated by the pulverizer (3) and about 50% of the primary air, and each of the primary-air/rich-portion nozzles (6) and the primary-air/thin-portion nozzles (7) are configured with perimeter airflow, and secondary air nozzles are configured to provide an offset airflow.
 8. The tangentially-fired furnace as recited in claim 7, wherein a first distance in a vertical direction between a specific primary-air/thin-portion nozzle (7) located at the top of the one of the corners and a specific primary-air/rich-portion nozzle (6) located at the bottom of the one of the corners is adjusted based on a second distance between the first burner and the second burner in the one of the corners. 